This invention relates to a power steering system, and more particularly to a power steering system including a rotary valve structure.
A conventional power steering system is generally constructed in such a manner as shown in FIGS. 1 to 3. More specifically, the conventional power steering system includes a gear box 1, an input shaft 2 inserted into the gear box 1, a pinion shaft 3 and a torsion bar 4 for connecting the input shaft 2 and pinion shaft 3 to each other. The input shaft 2 is integrally formed with a rotary spool 5, on which a sleeve 6 is fitted so as to be rotatable relative to each other. The so-arranged rotary spool 5 and sleeve 6 constitute a rotary valve structure. The sleeve 6 is adapted to be rotated integral with the pinion shaft 3, and the pinion shaft 3 is formed with a pinion 7, which is engaged with a rack 8.
The input shaft 2 is operatively associated with a steering wheel (not shown), resulting in being rotated with rotation of the steering wheel in any one of clockwise and counterclockwise directions. Irrespective of such rotation of the input shaft 2, the pinion shaft 3 is kept non-rotated because grounding resistance acts on the side of the pinion shaft 3. Thus, the input shaft 2 and pinion shaft 3 are caused to rotate relative to each other while twisting the torsion bar 4. The relative rotation between both input shaft 2 and pinion shaft 3 causes the rotary spool 5 and sleeve 6 to be rotated relative to each other, so that the rotary valve structure may be changed over.
FIG. 2 shows an equivalent circuit for the rotary valve structure and FIG. 3 is a cross-sectional view of the equivalent circuit which is taken in a direction perpendicular to the axial direction of the power steering system. As shown in FIGS. 2 and 3, the rotary valve structure comprises a first valve group V.sub.1 and a second valve group V.sub.2 each arranged so as to control fluid guided from a pump P to a power cylinder PC and a third valve group V.sub.3 and a fourth valve group V.sub.4 each arranged so as to control fluid guided from the pump P to a reservoir tank T. At the boundary between the first and second valve groups V.sub.1 and V.sub.2 and the third and fourth valve groups V.sub.3 and V.sub.4 are provided block means 63 as shown in FIG. 3, to thereby prevent communication between the respective valve groups.
Now, supposing that the rotary valve structure is changed over in one direction, valves a.sub.1 and d.sub.1 of the first valve group V.sub.1 and valves a.sub.2 and d.sub.2 of the second valve group V.sub.2 are open, and valves b.sub.1 and c.sub.1 of the first valve group V.sub.1 and valves b.sub.2 and c.sub.2 of the second valve group V.sub.2 are closed. This causes pressure fluid to be supplied to one of pressure chambers of the power cylinder PC designated at reference numeral 9 and fluid in the other pressure chamber 10 to be returned to the reservoir tank T.
Also, in the third valve group V.sub.3, a valve e.sub.3 and a valve h.sub.3 each are set to have a small degree of opening and a valve g.sub.3 and a valve f.sub.3 each are set to have a large degree of opening; whereas in the valve group V.sub.4, a valve e.sub.4 and a valve h.sub.4 each are set to have a small degree of opening and a valve g.sub.4 and a valve f.sub.4 each are set to have a large degree of opening. This causes fluid flowing into the third and fourth valve groups V.sub.3 and V.sub.4 to be returned to the reservoir tank T.
When the steering wheel is rotated in a direction opposite to the above-described direction, the valves b.sub.1 and c.sub.1 of the first valve group V.sub.1 and the valves b.sub.2 and c.sub.2 of the second valve group V.sub.2 are open and the valves a.sub.1 and d.sub.1 of the first valve group V.sub.1 and the valves a.sub.2 and d.sub.2 of the second valve group V.sub.2 are closed. This causes pressure fluid to be fed to the other pressure chamber 10 of the power cylinder PC and fluid in the one pressure chamber 9 to be returned to the reservoir tank T.
Also, in the third valve group V.sub.3, the degree of opening of each of the valve f.sub.3 and g.sub.3 is reduced and that of each of the valve e.sub.3 and h.sub.3 is increased. In the fourth valve group V.sub.4, the degree of opening of each of the valves f.sub.4 and g.sub.4 are decreased and that of each of the valves e.sub.4 and h.sub.4 is increased. This results in working fluid flowing into the third and fourth valve groups V.sub.3 and V.sub.4 being returned to the reservoir tank T.
On the return side of the third and fourth valve groups V.sub.3 and V.sub.4 is arranged a passage 11, which is merged in the form of an external piping into an external piping passage 12 on the return side of the first and second valve group V.sub.1 and V.sub.2 and provided with a variable throttle valve 13a. The variable throttle valve 13a is electrically connected to a controller C. The controller C is adapted to generate a signal depending upon traveling conditions such as the speed of a vehicle or the like to control the degree of opening of the variable throttle valve 13a. More particularly, the controller C functions to increase the opening of the variable throttle valve 13a as the speed is increased and reduces it as the speed is decreased.
An increase in degree of opening of the variable throttle valve 13a causes a ratio of the amount or flow rate of fluid returned to the reservoir tank T to the amount or flow rate of fluid discharged from the pump P to be increased, so that the amount or flow rate of fluid fed to the power cylinder PC is reduced relatively or correspondingly, leading to a decreased in power assisting force.
On the contrary, a decrease in opening of the variable throttle valve 13a causes the amount or flow rate of fluid returned to the reservoir tank T to be reduced correspondingly, so that the amount or flow rate of fluid supplied to the power cylinder PC is increased, resulting in power assisting force being increased correspondingly.
As described above, the conventional power steering system is so constructed that power assisting force for the power cylinder PC is controlled depending upon the speed of a vehicle and in any case, the amount of oil returned to the reservoir tank T is adjusted depending upon the opening of the variable throttle valve 13a and the supply pressure to the power cylinder PC is finally controlled through restriction of the first to fourth valve groups V.sub.1 to V.sub.4 . Such construction of the conventional system causes pressure acting on the third and fourth valve groups positioned on the upstream side of the variable throttle valve 13a to be substantially equal to that acting on the first and second valve groups and to be distributed and act on the rotary spool 5 in a circumferentially equal manner, resulting in preventing the rotary valve structure from being deformed.
Unfortunately, in the conventional power steering system constructed as described above, the variable throttle valve 13a connected to the gear box 1 is arranged away from the gear box 1 as shown in FIG. 1, so that it is required to arrange a piping, a coupling, and the like between the variable throttle valve 13a and the gear box 1 in order to connect the variable throttle valve 13a to the gear box 1, resulting in the power steering system being complicated in structure. Also, this causes the power steering system to be highly large-sized because a space for piping and the like is required.
Further, the arrangement of the piping, coupling and the like requires to form a connection between the piping and the coupling with threads, leading to an increase in the number of working steps and therefore an increase in manufacturing cost.